The present invention relates to an antenna for radiating and receiving circular polarised electromagnetic signals, in particular signals in the microwave or mm-wave frequency range.
The recent developments in commercial microwave and millimeter-wave communication systems are tremendous. Possible mass market applications are broad band home networks, wireless LANs, private short radio links, automotive mm-wave radars, microwave radio and TV distribution systems (transmitters and ultra low cost receivers). Particularly, the frequency band of 59 to 64 GHz is becoming very important for short range high data rate communication in respect of a large variety of practical applications starting from very high data rate WLANs to HD video transmission for indoor applications. Due to the possible mass market introduction of hand-held devices for these applications, a need for cheap and effective circular polarised antennas with high gain exists. Circular polarised antennas have the principal advantage, that no need for a proper orientation of the antenna is necessary, unlike linear polarised antennas, so that circular polarised antennas only need to be pointed to the direction of the data transmission. Moreover, if reflected transmission waves are approaching the receiver, these reflected waves have a changed polarisation compared to the waves of the not reflected main path. Thus, more simple modulation schemes are possible particularly for the 60 GHz operation range.
Circular polarised antennas with dipole means for radiating and receiving electromagnetic signals are known in many different variations. E.g. K. Hirose, K. Kawai, H. Nakano xe2x80x9cAn array antenna composed of outer-fed curl elementsxe2x80x9d IEEE AP-S 1998, 0-7803-4478-2/98 describe an antenna with more than one spiral shaped element attached to one feed line. The proposed antenna structure has the disadvantage, that a full multi element high gain beam antenna cannot be realised on the basis of the proposed approach. For the feeding of the antenna, a microstrip line is proposed and the dipole portions of the antennas are displaced and do not have a feeding point at the same location. Generally, the solution proposed in this article suffers from the disadvantage of a small operation bandwidth and a small axial ratio bandwidth and further that a high gain operation and a planar feeding of the antenna structure is not possible.
R. Ramirez, N. Alexopoulos xe2x80x9cSingle proximity feed microstrip alchimedean spiral antennasxe2x80x9d IEEE AP-S 1998, 0-7803-4478-2/98 propose circular polarised antenna elements with spiral shape dipole structures, whereby the feeding of the spiral shaped radiating elements is done in the middle of the elements. Although spiral shape elements fed in the middle are known for providing a high operation bandwidth, this kind of feeding has the drawback of a large geometrical size and very limited gain.
The object of the present invention is therefore to provide a circular polarised antenna with a dipole means comprising a first and a second element for radiating and receiving electromagnetic signals, whereby the first and the second element have a spiral shape, which can be manufactured in a simple and cost effective way and which can be operated with a high gain.
The above object is achieved by an antenna according to claim 1, comprising a dielectric substrate comprising a front and a back dielectric face, at least one dipole means comprising a first and a second element for radiating and receiving electromagnetic signals, said first element being printed on said front face and said second element being printed on said back face, said first and said second element having a spiral shape, respectively, both spirals being open, and metal feeding means for supplying signals to and from said dipole means, said metal feeding means comprising a first line printed on said front face and coupled to said first element at a first feeding point and a second line printed on said back face and coupled to said second element at a second feeding point, said first and said second feeding point overlapping each other.
The proposed new antenna is a circular polarised antenna which can be manufactured in a simple and very cost effective way and which can be operated with a high gain in the microwave and mm-wave range. Further, the proposed antenna structure allows a planar feeding which allows simple and easy transition and interface structures for the connection with other processing elements in the high frequency range. Further, the proposed antenna structure allows the integration of other high frequency integrated circuitry components on the same substrate, since the geometrical size of the dipole means is relatively small due to the spiral shape. Further, the proposed antenna geometry can be reproduced easily, which means that the manufacturing tolerances are not critical.
Advantageously, the spirals formed by the first and the second element have a constant radius. In other words, the spirals have a circular shape, so that each element forms a ring. Hereby, the spiral formed by the first and the second element may almost form a closed loop, respectively. One of the general features of the antenna according to the present invention is that the first and second feeding point couples the first and the second feeding line, respectively, to one end of each of the first and the second element, respectively. The other end of the first and the second element is a free or open end. Thus, the first and the second element almost forming a closed loop means that the free or open end of each of the elements is very close to the location were the first and the second feeding points are, but does not touch them.
Alternatively, the spirals by the first and the second element having a constant radius respectively form less than one complete turn.
In an alternative advantageous example of the antenna according to the present invention, the spirals formed by the first and the second element respectively have a decreasing radius toward their respective open end. This means that the radius of the spiral at the beginning, i.e. close to the respective feeding point, is larger and decreases towards the open end of the respective element. Hereby, the spirals formed by the first and the second element, respectively, may advantageously form less than one, one or more than one complete turn depending on the required size and application.
Further advantageously, the width of the first and the second element, respectively, decreases from the respective feeding point towards the respective open end of the spirals. Alternatively, it might by advantageous if the width of the first and the second element, respectively, increases from the respective feeding point towards the respective open end of the spirals.
Further advantageously, the first and the second line of the metal feeding means may be balanced microstrip lines.
Further advantageously, the first and the second line of the metal feeding means extend beyond the respective feeding point.
Further advantageously, a reflector means may be provided, which is spaced to and parallel with the back face of the dielectric substrate, with a low loss material being located between the reflector means and the back face. Hereby, the reflector means are advantageously spaced from the middle of the substrate by a quarter wavelength of the centre frequency of operation of the antenna.
The present invention further provides a phased antenna array comprising a plurality of antennas or antenna elements as described above, whereby the metal feeding means of the antennas are connected to metal transmission structures, respectively printed on the front face and the back face of the dielectric substrate. Hereby, the transmission structures are advantageously balanced and respectively comprise tapered microstrip lines. The tapered microstrip lines advantageously provide improved impedance matching. Further advantageously, a plurality of holes are provided in the substrate. The holes in the substrate on locations were no first and second elements and metal feeding means are printed increase the axial ratio quality of the antenna, whereby at the same time the low cost manufacturing process can be maintained.